Growth and regrowth of an autogenous rat sarcoma: effects of nitrogen mustard

Immediate and delayed effects of nitrogen mustard (HN2) (0.1 mg/kg/day for 4 days) on the growth and cell proliferation patterns of the 3-methylcholanthrene-induced autogenous rat sarcoma were studied. Tumor cells were labeled continuously with 0.5 muCi tritiated thymidine/g for 24 hours. The labeling index fell from 36.4 to 14.0% and the mitotic index from 0.88 to 0.67% after two treatments with HN2. At that time, tumor growth stopped and remained arrested during HN2 administration. After four injections of HN2, the labeling index was reduced further to 0.73% and the mitotic index to 0.36%. After the drug was withdrawn, tumor growth resumed at the pretreatment rate, even though the labeling index on day 3 was only 15.5% (or 40% of the control). The percent labeled mitosis curves and DNA contents, before and 4 days after HN2 was given, were similar. It was concluded that a subpopulation of cells of predominantly short intermitotic times caused tumor growth before and after drug treatment.